The demand for materials exhibiting exceptional hardness, durability and stability at very high temperatures (in excess of 1500.degree. C.) has led to renewed interest in refractory materials, such as carbides, borides, beryllides, nitrides and silicides. For example, ultra-refractory ceramic-ceramic composites, especially silicides, may exhibit sufficient oxidation resistance to be used as structural components at elevated temperatures. Potential applications for ultra-refractory ceramic-ceramic composites include energy-related technologies, such as internal combustion and jet engines and heat exchangers, and metal cutting and forming technologies, such as cutting tools and dies. One of the major challenges to be surmounted if ceramic-ceramic composites are to be widely used as high temperature structural materials is the difficulty and cost associated with processing the materials into dense complex shapes.
An example of a refractory material having a high melting point, hardness and electrical conductivity is titanium diboride (TiB.sub.2). J. Ramberg et al., "Resistance of TiB.sub.2 to High Temperature Yielding", 68 J. Am. Cer. Soc'y C78-C79 (1985) discloses that ultrapure fine grain TiB.sub.2 displays extraordinary resistance to plastic deformation at elevated temperatures because of the high Peierls stress intrinsic to the material. Titanium diboride has a fracture toughness of about 5 MPa m.sup.1/2. In contrast, titanium carbide (TiC) deforms plastically at 1000.degree. to 1500.degree. C., as discussed by W. Williams, "Influence of Temperature, Strain Rate, Surface Condition and Composition on the Plasticity of Transition-Metal Carbide Crystals" 35 J. Appl. Phys. 1329-38 (1964). Williams discloses, however, that TiC can be precipitation hardened by TiB.sub.2 at high temperatures by the addition of a fraction of a percent of boron. This combination of extreme resistance to plastic deformation, precipitation hardening and the high temperature plasticity intrinsic to the TiC phase suggests that TiB.sub.2 --TiC composites would be attractive high-temperature structural ceramics. In the TiB.sub.2 --TiC system, the eutectic temperature is greater than 2600.degree. C.
Ideally, materials should be processed in a soft, ductile condition and then strengthened by heat treatment, an approach which is fundamental in metallurgy. This concept has also been applied to ceramic materials. In the glass-ceramic industry, parts are formed and shaped from molten material and subsequently heat treated to crystallize the glass into a more refractory and tougher material. In transient liquid phase sintering, the liquid phase solidifies by diffusional homogenization during sintering, as discussed in R. German, Liquid Phase Sintering, Plenum Press, (New York 1985). Recently transient viscous sintering has been shown to facilitate the fabrication of mullite and mullite composites, as discussed in M Sacks et al., "Fabrication of Mullite and Mullite-Matrix Composites by Transient Viscous Sintering of Composite Powders", 74(10) J. Am. Cer. Soc'y 2428-37 (1991). In each of the aforementioned processes, a viscous, easily deformable phase is formed during fabrication, after which the phase is removed by crystallization or reaction.
Since carbides, borides, beryllides, nitrides and silicides have high melting temperatures, they are extremely hard to process. Attempts to fabricate monolithic and composite materials have involved particulate processing at very high sintering temperatures. However, such processes are generally incapable of achieving full density. For example, when a mixture of TiC and TiB.sub.2 powder is hot pressed at 2200.degree. C. at a pressure of 10 MPa, only 90% of theoretical full density is achieved, as disclosed by I. Spivak et al., "Creep in the Binary System TiB.sub.2 --TiC and ZrB.sub.2 --ZrN", 137 Sov. Powder Metall. Met. Cer. 617-20 (1974).
To attempt to reduce the difficulties in densification, several modifications of press and sinter technology and/or particulate processing alternatives have been evaluated. For example, ultra-fine plasma arc derived powders may be pressurelessly sintered at temperatures above 2000.degree. C., as disclosed by H Baumgartner et al., "Sintering and Properties of Titanium Diboride Made from Powder Synthesized in a Plasma-Ard Heater", 67 J. Am. Cer. Soc'y 207-12 (1984). The addition of relatively large amounts of sintering aids and liquid phase sintering in the processing of cemented carbides has also been suggested. More recently, ceramic-ceramic composites have been produced by direct reaction between a preform of a high temperature material and a liquid metal, e.g., B.sub.4 C and liquid Zr to yield ZrC--ZrB.sub.2, as disclosed by W. Johnson et al., "Preparation and Processing Platelet-Reinforced Ceramics by the Directed Reaction of Zirconium with Boron Carbide, "10(7-8) Cer. Eng. Sci. Proc. 588-98 (1989).
Generally, the formation of refractory compounds from the constituent elements occurs exothermically and exceptionally rapidly. These characteristics are exploited in the formation of a large number of materials by self-propagating high temperature synthesis, as discussed by J. Crider, "Self Propagating High Temperature Synthesis-A Soviet Method for Producing Ceramic Materials", 6(7-8) Cer. Eng. Sci. Proc. 520-29 (1985); J. McCauley et al., "Simultaneous Preparation and Self-Sintering of Materials in the System Ti--B--C" 3(9-12) Cer. Eng. Sci. Proc. 7 538-54 (1982); and U.S. Pat. Nos. 4,906,295 and 4,965,044.
U.S. Pat. No. 4,605,440 discloses boron-carbide-reactive metal composites prepared by (1) consolidation or preparation of the starting materials; (2) producing a capillarity-thermodynamic condition or wetting (forming a solid-liquid interface between) the starting materials; and (3) reacting or sintering the starting materials to produce the desired compositions.
U.S. Pat. Nos. 4,961,902 and 5,017,217 disclose methods for manufacturing ceramic/metal or ceramic/ceramic composite articles. The method includes the steps of (1) providing a mixture of first and second solid, particulate reactants in finely divided form; (2) heating the mixture up to a first temperature that is below the peritectic decomposition temperature of the second metal reactant at a moderately increasing rate; (3) heating the mixture through the peritectic decomposition temperature of the second metal reactant; (4) sintering the mixture; and (5) cooling the resulting reaction product. The first metal is a metal of Group IVb, Vb or VIb of the Periodic Table or mixtures thereof. The second metal compound must include a substance subject to peritectic decomposition, typically a boride compound.
The prior art processing methods discussed above are generally not capable of fabricating complex shaped ultra-refractory, fully dense, ceramic-ceramic composites at temperatures well below the melting temperatures of the components. Also, by eliminating the need for sintering aids as discussed below, the high temperature properties of the composites, such as creep resistance, would be greatly enhanced. The ability to incorporate other reinforcing agents, such as fibers or whiskers, into the composite to control strength and/or toughness is also desirable.
One of the more difficult problems associated with ceramics in general and ceramics to be used in structural applications in particular, is the fact that they are not easily joined to other parts. This places a severe limitation on their final use. The fabrication of graded composites, however, in which the side of the composite that would not be subjected to extreme temperatures and that is to be joined to other parts would be a metal and the other side, which is subjected to the extreme temperatures, is a ceramic-ceramic composite would be attractive. Another advantage of a graded structure lies in the fact that the metal outer skin may function as a ductile phase for blunting cracks in the refractory composite and prevent catastrophic failure.